Photoinhibition of photosynthesis was studied in young and mature detached sun needles of cypress under high irradiance (HI) of about 1 900 μmol m-2 s-1. The degree of photoinhibition was determined by means of the ratio of variable to maximum chlorophyll (Chl) fluorescence (Fv/Fm) and electron transport measurements. Compared with the mature needles, the young needles, containing about half the amount of Chl a+b per unit area, exhibited a higher proportion of total carotenoids (Car) as xanthophyll cycle pigments and had an increased ratio of Car/Chl a+b. The potential efficiency of photosystem (PS) 2, Fv/Fm, markedly declined in HI-treated young needles without significant increase of F0 level. In contrast, the Fv/Fm ratio declined with significant increase of F0 level in mature needles. In isolated thylakoids, the rate of whole chain and PS2 activity markedly decreased in young HI-needles in comparison with mature needles. A smaller inhibition of PS1 activity was observed in both needles. In the subsequent dark incubation, fast recovery was found in both needle Types that reached maximum PS2 efficiencies similar to those observed in non-photoinhibited needles. The artificial exogenous electron donors DPC, NH2OH, and Mn2+ failed to restore the HI-induced loss of PS2 activity in mature needles, while DPC and NH2OH significantly restored it in young needles. Hence, HI-inactivation was on the donor side of PS2 in young needles and on the acceptor side of PS2 in mature needles. Quantification of the PS2 reaction centre proteins D1 and 33 kDa protein of water splitting complex following HI-exposure of needles showed pronounced differences between young and mature needles. The large loss of PS2 activity in HI-needles was due to the marked loss of D1 protein of the PS2 reaction centre in mature needles and of the 33 kDa protein in young needles. and N. La Porta ... [et al.]
We investigated the effect of growth irradiance (I) on photon-saturated photosynthetic rate (Pmax), dark respiration rate (RD), carboxylation efficiency (CE), and leaf mass per unit area (LMA) in seedlings of the following four tropical tree species with contrasting shade-tolerance. Anthocephalus chinensis (Rubiaceae) and Linociera insignis (Oleaceae) are light-demanding, Barringtonia macrostachya (Lecythidaceae) and Calophyllum polyanthum (Clusiaceae) are shade-tolerant. Their seedlings were pot-planted under shading nets with 8, 25, and 50 % daylight for five months. With increase of I, all species displayed the trends of increases of LMA, photosynthetic saturation irradiance, and chlorophyll-based Pmax, and decreases of chlorophyll (Chl) content on both area and mass bases, and mass-based Pmax, RD, and CE. The area-based Pmax and CE increased with I for the light-demanders only. Three of the four species significantly increased Chl-based CE with I. This indicated the increase of nitrogen (N) allocation to carboxylation enzyme relative to Chl with I. Compared to the two shade-tolerants, under the same I, the two light-demanders had greater area- and Chl-based Pmax, photosynthetic saturation irradiance, lower Chl content per unit area, and greater plasticity in LMA and area- or Chl-based Pmax. Our results support the hypothesis that light-demanding species is more plastic in leaf morphology and physiology than shade-tolerant species, and acclimation to I of tropical seedlings is more associated with leaf morphological adjustment relative to physiology. Leaf nitrogen partitioning between photosynthetic enzymes and Chl also play a role in the acclimation to I. and Y.-L. Feng, K.-F. Cao, J.-L. Zhang.
We investigated the characteristics of gaseous exchanges and chlorophyll a fluorescence under different irradiances in two liana species Canavalia parviflora Benth. (Fabaceae) and Gouania virgata Reissk (Rhamnaceae), both of a semi-deciduous tropical forest of Southeast Brazil. We used cultivated plants growing under irradiances of 100, 40, 10, and 1.5 % of the photosynthetic photon flux density (PPFD). Higher net photosynthetic rates (PN) were observed during early morning under full sunlight. After this, reduced PN values were recorded due to pronounced stomatal closure. In Canavalia, the gas exchange responses diminished concomitant with reduced irradiance. Gouania exhibited a narrower range of response, with high PN values even at 10 % PPFD. Marked reduction of the effective photochemical yield (ΔF/Fm') near midday was observed, followed by increases in the non-photochemical quenching for both species under full sunlight. Despite the common occurrence of these species in open areas of the forest, both were able to maintain relatively high PN in shaded environments. We suggest that lianas present an intermediate physiological behaviour between shade and non-shade tolerant species. and M. C. Sanches, I. F. M. Válio.
Seasonal variability of maximum quantum yield of PSII photochemistry (Fv/Fm) was studied in needles of Taxus baccata seedlings acclimated to full light (HL, 100% solar irradiance), medium light (ML, 18% irradiance) or low light (LL, 5% irradiance). In HL plants, Fv/Fm was below 0.8 (i.e. state of photoinhibition) throughout the whole experimental period from November to May, with the greatest decline in January and February (when Fv/Fm value reached 0.37). In ML seedlings, significant declines of Fv/Fm occurred in January (with the lowest level at 0.666), whereas the decline in LL seedlings (down to 0.750) was not significant. Full recovery of Fv/Fm in HL seedlings was delayed until the end of May, in contrast to ML and LL seedlings. Fv/Fm was significantly correlated with daily mean (T mean), maximal (T max) and minimal (T min) temperature and T min was consistently the best predictor of Fv/Fm in HL and ML needles. Temperature averages obtained over 3 or 5 days prior to measurement were better predictors of Fv/Fm than 1- or 30-day averages. Thus our results indicate a strong light-dependent seasonal photoinhibition in needles of T. baccata as well as suggest a coupling of Fv/Fm to cumulative temperature from several preceding days. The dependence of sustained winter photoinhibition on light level to which the plants are acclimated was further demonstrated when plants from the three light environments were exposed to full daylight over single days in December, February and April and Fv/Fm was followed throughout the day to determine residual sensitivity of electron transport to ambient irradiance. In February, the treatment revealed a considerable midday increase in photoinhibition in ML plants, much less in HL (already downregulated) and none in LL plants. This suggested a greater capacity for photosynthetic utilization of electrons in LL plants and a readiness for rapid induction of photoinhibition in ML plants. Further differences between plants acclimated to contrasting light regimes were revealed during springtime de-acclimation, when short term regeneration dynamics of Fv/Fm and the relaxation of nonphotochemical quenching (NPQ) indicated a stronger persistent thermal mechanism for energy dissipation in HL plants. The ability of Taxus baccata to sustain winter photoinhibition from autumn until late spring can be beneficial for protection against an excessive light occurring together with frosts but may also restrict photosynthetic carbon gain by this shade-tolerant species when growing in well illuminated sites. and P. Robakowski, T. Wyka.